Vascular diseases include aneurysms which can rupture and cause hemorrhage, atherosclerosis which can cause the occlusion of the blood vessels, vascular malformation and tumors. Vessel occlusion or rupture of an aneurysm within the brain are common causes of stroke. Tumors fed by intra-cranial arteries can grow within the brain to a point where their mass and size can cause a stroke or the symptoms of stroke, requiring surgery for removal of the tumors or other remedial intervention.
Occlusion of the coronary arteries, for example, is a common cause of heart attack. Diseased and obstructed coronary arteries can restrict the flow of blood in the heart and cause tissue ischemia and necrosis. While the exact etiology of sclerotic cardiovascular disease is still in question, the treatment of narrowed coronary arteries is more defined. Surgical construction of coronary artery bypass grafts (CABG) is often the method of choice when there are several diseased segments in one or multiple arteries. Conventional open-heart surgery is, of course, very invasive and traumatic for patients undergoing such treatment. Therefore, alternative methods being less traumatic are highly desirable.
One of the alternative methods is balloon angioplasty that is a technique in which a folded balloon is inserted into a stenosis which occludes or partially occludes an artery and is inflated to open the occluded artery. Another alternative method is atherectomy that is a technique in which occlusive atheromas are cut from the inner surface of the arteries. Both methods suffer from reocclusion with certain percentage of patients.
One therapy for vascular occlusions is placement of an expandable metal wire-frame including stent, within the occluded region of blood vessel to hold it open. The stent is delivered to the desired location within a vascular system by a delivery means, usually a catheter. Some advantages of the stent-placement method over conventional vascular surgery include obviating the need for surgically exposing, removing, replacing, or by-passing the defective blood vessel, including heart-lung by-pass, opening the chest, and general anaesthesia.
When inserted and deployed in a vessel, duct or tract (“vessel”) of an organic body, for example a coronary artery after dilatation of the artery by balloon angioplasty, a stent acts as a prosthesis to maintain the vessel open. The stent usually has an open-ended tubular form with interconnected struts as its sidewall to enable its expansion from a first outside diameter which is sufficiently small to allow the stent to traverse the vessel to reach a site where it is to be deployed, to a second outside diameter sufficiently large to engage the inner lining of the vessel for retention at the site. A stent is typically delivered in an unexpanded state to a desired location in a body lumen and then expanded. The stent may be expanded via the use of a mechanical device such as a balloon, or the stent may be self-expanding.
Stents are typically made from biocompatible metals such as stainless steel, nickel-titanium, cobalt-chromium, tantalum, and the like, which provide sufficient hoop strength to perform the scaffolding function. Furthermore, stents have a minimal wall thickness in order to minimize blood flow blockage. Stents of various construction have been proposed, including the Palmaz-Schatz® balloon expandable metal stent, the Wallstent self-expanding braided metal stent, the Strecker knitted metal stent, the Instent® coil stent, the Cragg coiled stent and the Gianturco Z stent. However, stents can sometimes cause complications including thrombosis and neointimal hyperplasia, such as by inducement of smooth muscle cell proliferation at the site of implantation of the stent.
A factor affecting the choice of stent and stent material is allergic reaction to common stent materials suffered by a statistically significant percentage of the patient population subjected to stenting. These materials include chrome, nickel, and medical 316L stainless steel containing about 16% nickel.
Another consideration in material selection is the need for a physician to be able to visualize the position of a stent during implantation to the desired target site in an organic body, and for purpose of examination from time to time thereafter at the implant site, typically by X-ray fluoroscopy. The sidewall of the stent must be sufficiently thick, depending on the stent material, not only to withstand the vessel wall recoil that invariably follows deployment at the target site, but to allow the stent to be seen on the fluoroscope. Various materials, such as 316L stainless steel, possess suitable mechanical strength, however the X-ray absorption of the metal is low. The fluoroscopic visibility of stainless steel, for example, in a thickness below 100 μm is very poor because of the limited extinction of x-rays by such a thin metal tube. In addition, increasing the sidewall thickness of a stent to enhance its radiopacity and recoil resistance makes the stent less flexible. On the other hand, the thinner the stent strut, the less the lumen of the stented vessel is obstructed. Moreover, a thin stent is covered more readily by a neoendothelial build-up. Thus, it is desirable to make the sidewall of a stent as thin as can be practically achieved, without compromising the strength or visibility of the stent.